What is the easiest way to write dimensional expression?

Question

I want to do the following arithmetic:

(x-y)*unit/s where x, y, s are rational constants and unit is in length unit.

\documentclass{article}
\usepackage{calc}
\newlength\unit
\unit=16pt
\def\x{0.5}
\def\y{0.3}
\def\s{0.8}

\newlength\temp
\begin{document}
\temp=\unit/\real{\s}*(\real{\x}-\real{\y})\relax
\the\temp
\end{document}

What is the easiest way to write dimensional expression?

Answer 1

To be honest the easiest way is to calculate it using PGF.

\documentclass{article}
\usepackage[demo]{graphics}
\usepackage{pgf}
\newlength\unit
\unit=16pt
\def\x{0.5}
\def\y{0.3}
\def\s{0.8}
\newlength\temp
\begin{document}
\makeatletter
\pgfmathsetlength{\temp}{\unit/\s*(\x-\y)}
\the\temp
\includegraphics[width=\temp]{test}
\end{document}

If you pass it to \includegraphics, always better as a macro parameter.

 \def\putgraphic#1{%
   \includegraphics[viewport=#1 10 20 20]{test.png}}
 \putgraphic{\temp}

Answer 2

Here's with expl3 and the fp module (not the expandable version):

\documentclass{article}
\usepackage{xparse}
\ExplSyntaxOn
\fp_new:N \l_forgiver_factor_fp
\NewDocumentCommand{\xys}{ O{\funit} m m m }
 {
  \fp_set:Nn \l_forgiver_factor_fp { #2 }
  \fp_sub:Nn \l_forgiver_factor_fp { #3 }
  \fp_div:Nn \l_forgiver_factor_fp { #4 }
  #1 = \fp_use:N \l_forgiver_factor_fp #1
 }

\ExplSyntaxOff
\newlength\funit
\setlength\funit{16pt}

\begin{document}   
\xys{3.125}{2.175}{2.5}\the\funit
\end{document}

This will output

6.08008pt

that complies from echo "16*(3.125-2.175)/2.5" | bc -l, which actually prints

6.08000000000000000000

You can set other lengths by calling

\xyz[\mylen]{<x>}{<y>}{<s>}